Process to purify dialkyl sulfides

ABSTRACT

The invention relates to new processes to prepare low odor dialkyl sulfides, the low odor dialkyl sulfides obtainable by these processes and to methods of using these low odor dialkyl sulfides. Moreover, the invention relates to a process to prepare dialkyl sulfide borane complexes of high purity, the dialkyl sulfide borane complexes obtainable by this process and to a process for enantioselective reductions employing these dialkyl sulfide borane complexes of high purity as reducing agent.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. §371)of PCT/EP2009/058848, filed Jul. 10, 2009, which claims benefit of U.S.application 61/080,868, filed Jul. 15, 2008.

FIELD OF THE INVENTION

The invention relates to new processes to purify dialkyl sulfides, thepurified dialkyl sulfides obtainable by these processes and to methodsof using these purified dialkyl sulfides. Moreover, the inventionrelates to a process to prepare dialkyl sulfide borane complexes of highpurity, the dialkyl sulfide borane complexes obtainable by this processand to a process for enantioselective reductions employing these dialkylsulfide borane complexes of high purity as reducing agent.

BACKGROUND OF THE INVENTION

Dialkyl sulfides are versatile reagents for organic syntheses. Moreover,lower dialkyl sulfides are often employed as valuable solvents. Mostorganic sulfides with low molecular mass have intense unpleasant odors,which in many cases is further deteriorated through the presence ofsulfur-containing impurities (K.-M. Roy, “Thiols and Organic Sulfides”,Ullmann's Encyclopedia of Industrial Chemistry, 7^(th) Ed., pages 1 to28, Wiley-VCH Verlag GmbH & Co. KGaA 2008). For example, commercialdimethyl sulfide (DMS) contains highly malodorous impurities such ascarbon disulfide, carbonyl sulfide, methyl thiol, dimethyldisulfide,hydrogen sulfide and other sulfurous compounds at low levels. Eventhough the concentration of these impurities is less than one percentand for some compounds is less than 200 ppm, the odor of the impuritiesgives the DMS a vile stench of rotten eggs, cabbage and skunk. Somesulfur compounds having the odor of garlic or onions may also contributeto the odor. DMS of high purity has the odor of corn or a grassy meadow(D. J. Rowe, Perfumer & Flavorist 1998, Vol. 23, pages 9 to 14).

DMS is used as an anti-coking agent in petroleum steamcrackers and asraw material to make the solvent dimethylsulfoxide (DMSO). DMS is alsoused as a solvent and for coordination to metal compounds or other Lewisacids, i.e. borane compounds. Dimethyl sulfide borane (DMSB) is a stableconcentrated (10M) form of borane (BH₃) utilized in the pharmaceuticalindustry for the reduction of carbonyl compounds, imines, andhydroboration of double bonds in alkenes or alkynes. The unpleasant odorof DMSB made from impure DMS can be noticed at very low levels inproduction operations and can drift in the wind into communities.Responsible companies do not want to subject employees or neighboringpopulations to the odor from using DMS or DMSB.

The purification of DMS has been addressed in JP 49006287 by a steamdistillation process and separation of water from the dialkyl sulfide.This method is inappropriate for DMSB preparation due to residual waterin the DMS.

U.S. Pat. No. 6,736,879 discloses an absorption method to remove carbondisulfide from dimethyl sulfide. The absorption media is an activatedalumina treated with alkali metal and alkaline earth compounds and canbe regenerated. However, absorption of the impurities in dialkylsulfides may remove some contaminates but will not completely remove thevariety of impurities due to the reversibility of the absorptionprocess.

Some manufacturers of dimethyl sulfide and dimethyldisulfide have usedodor-masking compounds to give the mixture a more pleasant odor (U.S.Pat. No. 5,559,271, U.S. Pat. No. 6,639,110). The compounds used havefunctional groups that are reactive with borane and therefore cannot beused in the application of DMSB preparation.

Removal of hydrogen sulfide, carbon disulfide, and thiols (collectivelycalled acid gases) from gas streams and hydrocarbon mixtures has beentried with a number of methods, such as passing the gas through a packedbed calcinator (U.S. Pat. No. 6,136,144) or water and bromine (U.S. Pat.No. 5,433,828), absorption with nitrogen containing heterocycles (DE19828977, U.S. Pat. No. 5,480,860) or reversible absorbents (U.S. Pat.No. 4,173,619, US 2005/0205470). Hydrocarbon purification to removesulfurous and phosphine components prior to polymerization has usedalkali metals on supports (U.S. Pat. No. 5,302,771, U.S. Pat. No.6,124,410) or ion exchanged zeolites (U.S. Pat. No. 4,358,297). Otherexamples use amines on a solid support (U.S. Pat. No. 4,999,175),transition metal oxides (U.S. Pat. No. 5,157,201) or reaction with aGroup 1B metal halide amine (U.S. Pat. No. 5,382,417).

Removal of alkyl sulfides and thiols in plant effluent by oxidation hasbeen addressed (U.S. Pat. No. 6,015,536, U.S. Pat. No. 6,277,344, U.S.Pat. No. 5,439,641). Oxidation is not applicable to DMS purificationbecause the DMS would also be oxidized.

High purity DMS is a desirable commercial product suitable as flavoringagent or solvent as well as low odor compounds made from it, i.e. DMSOor DMSB. Selective removal of undesirable components, while leaving thedesired dimethyl sulfide, is not adequately addressed by currentliterature. Furthermore, oxidation methods are destructive to allcomponents of the mixture. It is highly desirable to remove malodorousimpurities from DMS while leaving the DMS unchanged.

It was therefore an object of the present invention to develop processesfor the purification of dialkyl sulfides in order to provide low odordialkyl sulfides, i.e. with an odor that is less noticeable compared tocommercially available dialkyl sulfides.

SUMMARY OF THE INVENTION

Accordingly, new processes to purify dialkyl sulfides have beendeveloped, comprising the step of bringing a dialkyl sulfide intocontact with at least one base and/or at least one alkali or alkalineearth metal.

Moreover, low odor dialkyl sulfides have been developed that lack majormalodorous impurities and methods of using these low odor dialkylsulfides to prepare low odor products or as low odor solvent.

Furthermore, dialkyl sulfide borane complexes of high purity togetherwith a process for their preparation have been developed and a improvedprocess for enantioselective reductions employing these dialkyl sulfideborane complexes of high purity as reducing agent was found.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the present invention is a process to purify dialkylsulfides comprising the step of bringing a dialkyl sulfide into contactwith at least one base and/or at least one alkali or alkaline earthmetal, wherein the base is a metal alcoholate, a metal oxide, a metal oralkylammonium hydroxide, a metal or alkylammonium carbonate, a metalenolate, a metal amide or a metal hydride, wherein the metal is selectedfrom the group, consisting of the alkali metals, the alkaline earthmetals and the metals of the groups IIIa to VIIIa, Ib and IIb.

A preferred embodiment of the present invention is a process to purifydialkyl sulfides comprising the step of bringing a dialkyl sulfide intocontact with at least one base and at least one alkali or alkaline earthmetal.

According to the invention a dialkyl sulfide is a compound with thechemical formula R—S—R′, wherein R and R′ are independent from eachother C₁-C₁₈ alkyl, C₃-C₁₄ cycloalkyl, substituted C₁-C₁₈ alkyl,substituted C₃-C₁₄ cycloalkyl or R and R′ are connected as a divalenthydrocarbon moiety, that may contain further sulfur, oxygen, or nitrogenatoms, which together with the sulfur atom forms a cyclic dialkylsulfide structure.

In a preferred embodiment of the present invention the dialkyl sulfideis dimethyl sulfide, diethyl sulfide, methyl ethyl sulfide, methylisopropyl sulfide or thioxane.

As used in connection with the present invention, the term “C₁-C₁₈alkyl” denotes a branched or an unbranched saturated hydrocarbon groupcomprising between 1 and 18 carbon atoms; examples are methyl, ethyl,propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, amyl,isoamyl, sec-amyl, 1,2-dimethylpropyl, 1,1-dimethylpropyl, hexyl,4-methylpentyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl,1,1-dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl,1,2-dimethylbutyl, 1,3-dimethylbutyl, 1,2,2-trimethylpropyl,1,1,2-trimethylpropyl, heptyl, 5-methylhexyl, 1-methylhexyl,2,2-dimethylpentyl, 3,3-dimethylpentyl, 4,4-dimethylpentyl,1,2-dimethylpentyl, 1,3-dimethylpentyl, 1,4-dimethylpentyl,1,2,3-trimethylbutyl, 1,1,2-trimethylbutyl, 1,1,3-trimethylbutyl, octyl,6-methylheptyl, 1-methylheptyl, 1,1,3,3-tetramethylbutyl, nonyl, 1-, 2-,3-, 4-, 5-, 6- or 7-methyloctyl, 1-, 2-, 3-, 4- or 5-ethylheptyl, 1-, 2-or 3-propylhexyl, decyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- and 8-methylnonyl,1-, 2-, 3-, 4-, 5- or 6-ethyloctyl, 1-, 2-, 3- or 4-propylheptyl,undecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-methyldecyl, 1-, 2-, 3-,4-, 5-, 6- or 7-ethylnonyl, 1-, 2-, 3-, 4- or 5-propyloctyl, 1-, 2- or3-butylheptyl, 1-pentylhexyl, dodecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8-,9- or 10-methylundecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- or 8-ethyldecyl, 1-,2-, 3-, 4-, 5- or 6-propylnonyl, 1-, 2-, 3- or 4-butyloctyl and1-2-pentylheptyl. Preferred are the alkyl groups methyl, ethyl, propyl,isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, amyl, isoamyl,sec-amyl, 1,2-dimethylpropyl, 1,1-dimethylpropyl, hexyl and octyl.

The term “C₃-C₁₄ cycloalkyl” denotes a saturated hydrocarbon groupcomprising between 3 and 14 carbon atoms including a mono- or polycyclicstructural moiety. Examples are cyclopropyl, cyclobutyl, cyclopentyl,cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl or cyclodecyl. Preferredare the cycloalkyl groups cyclopropyl, cyclopentyl and cyclohexyl.

The terms “substituted C₁-C₁₈ alkyl” or “substituted C₃-C₁₄ cycloalkyl”denotes an alkyl group or a cycloalkyl group as defined above in whichat least one hydrogen atom is replaced by a fluorine atom.

Examples for cyclic dialkyl sulfides include thietane, thiolane,thioxane, 1,3-dithiane, 1,4-dithiane and 2-methyl-1,3-dithiane.

In a preferred embodiment of the present invention the dialkyl sulfideused as a starting material should have a purity of at least 95 wt.-%,preferably at least 99 wt.-%.

In another preferred embodiment of the present invention the dialkylsulfide and the at least one base and/or at least one alkali or alkalineearth metal are brought into contact for a time in the range from about1 second to about 24 hours, most preferred in the range between 1 minuteand 3 hours.

In one embodiment of the present invention the dialkyl sulfides isbrought into contact with at least one base, which is a metalalcoholate, a metal oxide, a metal or alkylammonium hydroxide, a metalor alkylammonium carbonate, a metal enolate, a metal amide or a metalhydride. The metal can be selected from the group, consisting of thealkali metals, the alkaline earth metals and the metals of the groupsIIIa to VIIIa, Ib and IIb. Preferred are the alkali metals, magnesium,calcium, barium, scandium, titanium, vanadium, chromium, manganese,iron, cobalt, nickel, copper and zinc.

A metal alcoholate comprises at least one alkoxy group that can bederived from any branched or unbranched aliphatic alcohol. Examples ofsuitable metal alcoholates that can be employed in the present inventioninclude potassium alcoholates, sodium alcoholates, lithium alcoholates,magnesium alcoholates, zinc alcoholates and titanium alcoholates derivedfrom methanol, ethanol or t-butanol.

In another embodiment of the present invention the dialkyl sulfides isbrought into contact with at least one alkali or alkaline earth metal.These metals can also be employed as mixtures with each other (i.e. asalloys) or with mercury (i.e. as amalgams). Preferred are dispersions ofmetals as well as metals on a supporting media such as alumina, silica,diatomaceous earth, graphite or other common mineral compositions. Mostpreferred is the use of alkali metals, especially of sodium-potassiumalloys.

In cases where the metal employed does not only react with theimpurities but also with the dialkyl sulfide to be purified a largeexcess of the metal should be avoided, otherwise the overall yield wouldbe reduced.

In a preferred embodiment of the present invention the dialkyl sulfideis brought into contact with a metal alcoholate and optionally with atleast one alkali or alkaline earth metal.

In one embodiment of the present invention the dialkyl sulfides isbrought into contact with at least one base and with at least one alkalior alkaline earth metal consecutively in separate steps. In a preferredembodiment of the present invention the dialkyl sulfides is brought intocontact with at least one base and with at least one alkali or alkalineearth metal simultaneously in one step.

In another preferred embodiment of the present invention the dialkylsulfide is brought into contact with potassium tert.-butoxide and asodium-potassium alloy.

Furthermore, the process of the present invention can be carried out inthe liquid or in the gas phase. For a commercial process, the method ofchoice for high throughput would be to subject gaseous dialkyl sulfideto a fixed bed containing a metal and a base on a supporting media toallow for sufficient contact time in order to irreversible remove themalodorous components from the dialkyl sulfide.

By application of the processes disclosed in this invention to crudedialkyl sulfides essentially all of the malodorous impurities will beconverted into non-volatile and perhaps insoluble derivatives. Thesederivatives can be removed from the dialkyl sulfide by any separationmethod known in the art. For example, precipitates of insolublederivatives can be filtered. Reaction products with a considerablesolubility in the dialkyl sulfide can be removed by distillation orextraction methods.

However, distillation of dialkyl sulfides can be problematic if carbondisulfide (CS₂) shall be removed. CS₂ forms an azeotrope with e.g.dimethyl sulfide (DMS), therefore distillation methods may notsatisfactorily remove this contaminant. Reaction of carbon disulfidewith a metal alcoholate generates a xanthate salt (Dunn, A. D.; Rudorf,W. Carbon Disulphide in Organic Chemistry; Ellis Horwood: Chichester1989, p. 316), but this reaction is reversible under distillationconditions. Further reaction of the xanthate salt with an electrophileconverts the salt to the xanthate ester which is stable and can easilybe separated from the dialkyl sulfide by distillation. Since any thioatesalt will also react with the electrophile to give the corresponding(and undesired) dialkyl sulfide, the electrophile should be chosen suchthat the products are non-volatile or high boiling.

Therefore, another embodiment of the present invention is a process topurify dialkyl sulfides comprising the steps of

-   -   a) bringing a dialkyl sulfide into contact with at least one        base and/or at least one alkali or alkaline earth metal, wherein        the base is a metal alcoholate, a metal oxide, a metal or        alkylammonium hydroxide, a metal or alkylammonium carbonate, a        metal enolate, a metal amide or a metal hydride, wherein the        metal is selected from the group, consisting of the alkali        metals, the alkaline earth metals and the metals of the groups        IIIa to VIIIa, Ib and IIb, and    -   b) reacting the product of step a) with an electrophile, and    -   c) distilling out the dialkyl sulfide.

In a preferred embodiment of the present invention the dialkyl sulfideis brought into contact in step a) with a metal alcoholate andoptionally with at least one alkali or alkaline earth metal.

As electrophile any organic halide, methanesulfonate,trifluormethanesulfonate, p-toluenesulfonate and the like can be used.For the reason mentioned above, the organic residue of the electrophileshould have a high molecular mass in order to generate high-boilingproducts. Examples of electrophiles suitable for the purification oflow-boiling dialkyl sulfides include octyl bromide, benzyl bromide andbenzyl chloride.

The new processes disclosed in this invention provide easy access to lowodor dialkyl sulfides, which are of great value for the production oflow odor compounds made from it, e.g. dimethyl sulfoxide or dimethylsulfide borane complex from dimethyl sulfide.

Another embodiment of the present invention is the dialkyl sulfideobtainable by one of the processes disclosed above.

In the dialkyl sulfides according to the invention the content of eachof the impurities carbon disulfide, methanethiol and dimethyldisulfideis below 0.01% wt. respectively.

Still another embodiment of the present invention is the method of usingthese low odor dialkyl sulfides to prepare low odor products or as a lowodor solvent.

A preferred embodiment of the present invention is a method of usingpurified dialkyl sulfides to prepare dialkyl sulfide borane complexes ofhigh purity. An even more preferred embodiment of the present inventionis a method of using purified dimethyl sulfide to prepare dimethylsulfide borane (DMSB) of high purity.

A further embodiment of the present invention is a process to preparedialkyl sulfide borane complexes of high purity comprising the steps of

-   -   a) bringing a dialkyl sulfide into contact with at least one        base and/or at least one alkali or alkaline earth metal, wherein        the base is a metal alcoholate, a metal oxide, a metal or        alkylammonium hydroxide, a metal or alkylammonium carbonate, a        metal enolate, a metal amide or a metal hydride, wherein the        metal is selected from the group, consisting of the alkali        metals, the alkaline earth metals and the metals of the groups        IIIa to VIIIa, Ib and IIb of the periodic table of elements, and    -   b) optionally reacting the product of step a) with an        electrophile and distilling out the dialkyl sulfide, and    -   c) reacting the dialkyl sulfide purified according to step a)        and optionally step b) with diborane.

A preferred embodiment of the present invention is a process to preparedimethyl sulfide borane (DMSB) of high purity by the process describedabove wherein dimethyl sulfide is employed as dialkyl sulfide.

The reaction of purified dialkyl sulfide with diborane according to stepc) of the process described above is usually carried out at temperaturesbetween −10 and +50° C., preferrably at ambient temperature. Diborane ispreferably employed in gaseous form and the reaction is preferablycarried out in a pressurized vessel.

Another embodiment of the present invention is the dialkyl sulfideborane complex of high purity obtainable by the process disclosed above.

In the dialkyl sulfide borane complexes according to the invention thecontent of each of the impurities carbon disulfide, methanethiol anddimethyldisulfide is below 0.01% wt. respectively.

It was found that the enantiomeric excess obtained in enantioselectivereductions with dialkyl sulfide boranes as reducing agent is higher withdialkyl sulfide borane complexes of high purity according to theinvention compared with regular dialkyl sulfide borane complexes. Thisimprovement is due to the lack of impurities like dimethyldisulfide(DMDS) or alkyl thiols (as shown by addition experiments, cf. Example 3)that are effectively removed by the purification processes disclosedabove.

Since for pharmaceutical products an enantiomeric excess of at least99.5% is required by regulations (Carey, J. S.; Laffan, D.; Thomson, C.;Williams, M. T. Organic & Biomolecular Chemistry 2006, 4, 2337),application of the dialkyl sulfide borane complexes of high purityaccording to the invention is very advantageous because it helps toavoid purification steps.

Therefore, another embodiment of the present invention is a process forenantioselective reductions which comprises using as reducing agent adialkyl sulfide borane complex of high purity according to theinvention.

Enantioselective reductions with dialkyl sulfide boranes as reducingagent are usually carried out in the presence of chiral boron-containingcatalysts liketetrahydro-1-methyl-3,3-diphenyl-1H,3H-pyrrolo[1,2-c][1,3,2]oxazaborole(known as the (R)- or (S)-MeCBS oxazaborolidine reagent, see Corey, E.J.; Helal, C. J. Angew. Chem. Int. Ed. 1998, 37, 1986) or chiralspiroborates like (R)- or(S)-2-[(1,3,2-dioxaborolan-2-yloxy)diphenylmethyl]pyrrolidine (seeOrtiz, M. M.; Stepanenko, V.; Correa, W.; Jesus, M. D.; Sandraliz, E.;Ortiz, L. Tetrahedron Asymmetry 2006, 17, 122).

In a preferred embodiment of the present invention the enantioselectivereduction is the reduction of a prochiral ketone or imine.

The following examples illustrate the present invention withoutlimitation of the same.

EXAMPLES Example 1

To see the affect of various treatments with alkali metals and/or baseson the odor of commercial dimethyl sulfide a series of experiments werecompleted. Impure dimethyl sulfide (about 99.0% DMS, Chevron-Phillips)was stirred with various reagents for 1 hour at ambient temperature inthe liquid phase. Based on odor detection by (4) human volunteers, theodor after contact with the reagents was determined and related to aparticular known odor.

TABLE 1 Odor assessment for dimethyl sulfide after different treatmentsEntry # Treatment Odor 1 No treatment Strong unpleasant odor 2 Sodiumhydroxide Lower odor but still rotten cab- bage 3 Potassiumtert.-butoxide (KTB) Lower odor but still rotten cab- bage 4 NaK StrongCS₂ odor (described as pine) 5 NaK and KTB/filtered Boiled sweet cornodor 6 NaK and KTB/distilled Trace odor of CS₂, pine 7 NaK andOctBr/distilled Only odor of sweet corn 8 NaK and BnBr/distilled Slightmusty odor 9 NaK and BnCl/distilled Only odor of sweet corn 10 NaK, KTBand OctBr/distilled Boiled sweet corn odor

The addition of sodium hydroxide (NaOH, entry 2, 1 g/10 ml DMS) orpotassium tert.-butoxide (KTB, entry 3, 1 g/10 ml DMS) to DMS caused anoticeable warming of the solution due primarily to the base dissolving.The residual odor of base treated samples was of rotten cabbage due todimethyldisulfide impurities remaining in the DMS. Addition of liquidsodium-potassium-alloy (NaK, entry 4, 0.2 g/10 ml DMS) showed visiblesigns of reaction by gas evolution (hydrogen) and a tan solid formed onthe beads of NaK. The odor after NaK treatment was still strongly ofcarbon disulfide. KTB was added to NaK treated DMS (entry 5, 1 g KTB+0.2g NaK/10 ml DMS) which greatly improved the odor. The odor of thisfiltered sample was of boiled sweet corn.

The NaK and KTB treated sample was distilled from the solids andremaining unreacted NaK. The DMS distillate (entry 6) still had a slightodor of carbon disulfide. Distillation of the DMS overcomes the problemof dissolved basic compounds, however due to the reversible nature ofthe reaction of carbon disulfide and base, the odor of distilled DMS hada hint of CS₂ odor. To adequately remove the traces of carbon disulfidean electrophile (bromooctane, benzyl bromide, benzyl chloride) was addedafter the addition of alkali metal and/or base to form the xanthateester and potassium bromide. After distillation the DMS produced by thismethod also had an odor of corn (entries 7, 9 and 10, amounts ofreagents are listed in example 2).

The most satisfactory odor removal was by contacting DMS with acombination of NaK and KTB.

Example 2

The following experimental procedure was used for DMS from two differentsources of dimethyl sulfide, Chevron-Phillips and Gaylord. DMS (100 g)was weighed into an oven dried round-bottom flask. NaK (0.5 g, 72 wt %K) was added to the sample and stirring commenced at room temperaturefor 1 h. One of three alkylating agents (1.96 g benzyl chloride, 2.65 gbenzyl bromide or 2.99 g 1-bromooctane) was then added. In each samplesome solid precipitate formed. The dimethyl sulfide was distilled fromthe solids at a boiling range of 36-37.5° C. The distilled samples werethen analyzed by GC/MS for purity, see Table 2 for results.

TABLE 2 Analytical data (GC/MS) for dimethyl sulfides after differenttreatments. Entry # Treatment Chevron-Phillips Gaylord 1 No treatment99.077 DMS 99.751 DMS 0.32 CS2 0.018 MeSH 0.024 MeSH 0.036 EtSMe 0.259DMDS 0.19 acetone 2 NaK and 99.829 DMS 99.885 DMS OctBr/distilled 0.00CS₂ 0.00 MeSH 0.00 MeSH 0.017 EtSMe 0.00 DMDS 0.02 acetone 0.019 octane0.018 OctBr 3 NaK and 99.76 DMS 99.889 DMS BnBr/distilled 0.00 CS₂ 0.00MeSH 0.00 MeSH 0.018 EtSMe 0.001 DMDS 0.079 acetone 4 NaK and 99.077 DMS99.922 DMS BnCl/distilled 0.00 CS₂ 0.00 MeSH 0.00 MeSH 0.018 EtSMe 0.002DMDS 0.042 acetone All figures are % wt.. DMS is dimethyl sulfide, CS₂is carbon disulfide, DMDS is dimethyldisulfide, MeSH is methanethiol,EtSMe is ethyl methyl sulfide, OctBr is 1-bromooctane, BnCl is benzylchloride and BnBr is benzyl bromide. Traces of acetone were fromcleaning of the syringe between samples

Example 3

The following experimental procedure was used for the enantioselectivereduction of acetophenone with DMSB of different purity in the presenceof a chiral catalyst:

High purity DMSB was prepared as a 10M solution by addition of gaseousdiborane at ambient temperature to DMS that has been purified accordingto the procedure disclosed in Table 2, Entry #4.

3.3 ml of a stock solution of the respective sulfide impurity in toluene(0.02 eq. impurity vs. acetophenone) were mixed with the high purityDMSB (6.24 mmol, 592 μl of the 10M DMSB solution, 0.75 eq. vs.acetophenone), the mixture was stirred at ambient temperatures for 60minutes, and then the chiral catalyst ((R)-MeCBS((R)-tetrahydro-1-methyl-3,3-diphenyl-1H,3H-pyrrolo[1,2-c][1,3,2]oxazaborole):167 μl, 0.167 mmol, 1M solution in toluene, 0.02 eq.; or (R)-DPP Spiro((R)-2-[(1,3,2-dioxaborolan-2-yloxy)diphenylmethyl]pyrrolidine): 53.81mg, 0.167 mmol, 0.02 eq.) was added. The mixture was agitated for 5minutes before a solution of acetophenone in toluene (834.5 mg, 8.32mmol in 3.2 ml toluene to result in 1M acetophenone solution duringreaction, 1 eq.) was added using a syringe pump within 10 minutes. Fiveminutes after the addition was completed, 0.3 ml of a sample was takenand hydrolyzed in 2M hydrogen chloride (2 ml). The top layer was dilutedwith toluene and investigated further by GC analysis for conversion andenantioselectivity. The reaction was repeated using DMSB prepared withDMS from conventional raw material sources such as from Chevron-Phillipsor Gaylord and the enantioselectivity was compared.

TABLE 3 Results for the enantioselective reduction of acetophenone withDMSB of different purity in the presence of a chiral catalyst. ImpurityDMSB Conversion Entry # Catalyst added Quality (%) ee (%) 1 (R)—MeCBS —Chevron- 99.86 97.6 Phillips 2 (R)—MeCBS — Gaylord 99.74 98.12 3(R)—MeCBS — High purity 99.6 99.04 4 (R)—MeCBS DMDS High purity 99.498.16 5 (R)—MeCBS EtSH High purity 99.86 94.67 6 (R)—DPP Spiro — Highpurity 99.9 99.02 7 (R)—DPP Spiro DMDS High purity 99.9 98.14 8 (R)—DPPSpiro EtSH High purity 99.8 96.46 DMSB is dimethyl sulfide borane, DMDSis dimethyldisulfide, EtSH is ethanethiol, (R)—MeCBS is(R)-tetrahydro-1-methyl-3,3-diphenyl-1H,3H-pyrrolo[1,2-c][1,3,2]oxazaborole,(R)—DPP Spiro is(R)-2-[(1,3,2-dioxaborolan-2-yloxy)diphenylmethyl]pyrrolidine

All the references described above are incorporated by reference in itsentirety for all useful purposes.

While there are shown and described certain specific structuresembodying the invention, it will be manifest to those skilled in the artthat various modifications and rearrangements of the parts may be madewithout departing from the spirit and scope of the underlying inventiveconcept and the same is not limited to the particular forms herein shownand described.

1.-22. (canceled)
 23. A process to purify a dialkyl sulfide comprisingbringing a dialkyl sulfide into contact with a metal alcoholate andoptionally with at least one alkali or alkaline earth metal.
 24. Theprocess according to claim 23, wherein the dialkyl sulfide is dimethylsulfide, diethyl sulfide, methyl ethyl sulfide, methyl isopropyl sulfideor thioxane.
 25. The process according to claim 23, comprising bringinga dialkyl sulfide into contact with at least one metal alcoholate and atleast one alkali or alkaline earth metal.
 26. The process according toclaim 23, wherein the dialkyl sulfide is brought into contact withpotassium tert-butoxide and a sodium-potassium alloy.
 27. The processaccording to claim 23, wherein the dialkyl sulfide is brought intocontact with at least one metal alcoholate and with at least one alkalior alkaline earth metal simultaneously in one step.
 28. A process topurify a dialkyl sulfide comprising the steps of a) bringing a dialkylsulfide into contact with at least one base and/or at least one alkalior alkaline earth metal, wherein the base is a metal alcoholate, a metaloxide, a metal hydroxide, alkylammonium hydroxide, a metal carbonate,alkylammonium carbonate, a metal enolate, a metal amide or a metalhydride, wherein the metal is an alkali metal, an alkaline earth metalor a metal of the groups IIIa to VIIIa, Ib and IIb of the periodic tableof elements, and b) reacting the product of step a) with an alkyl orbenzyl halide, and c) distilling out the dialkyl sulfide.
 29. Theprocess according to claim 28, wherein the dialkyl sulfide is broughtinto contact in step a) with a metal alcoholate and optionally with atleast one alkali or alkaline earth metal.
 30. A process to prepare adialkyl sulfide borane complex comprising the steps of a) bringing adialkyl sulfide into contact with at least one base and/or at least onealkali or alkaline earth metal, wherein the base is a metal alcoholate,a metal oxide, a metal hydroxide, alkylammonium hydroxide, a metal oralkylammonium carbonate, a metal enolate, a metal amide or a metalhydride, wherein the metal is selected from the group, consisting of thealkali metals, the alkaline earth metals and the metals of the groupsIIIa to VIIIa, Ib and IIb of the periodic table of elements, and b)reacting the product of step a) with an alkyl or benzyl halide, c)distilling out the dialkyl sulfide, and d) reacting the dialkyl sulfidepurified according to steps a) to c) with diborane.
 31. The processaccording to claim 30, wherein the dialkyl sulfide is dimethyl sulfide.32. A process for enantioselective reductions which comprises using asreducing agent the dialkyl sulfide borane complex obtained by a processaccording to claim
 30. 33. The process according to claim 32, whereinthe enantioselective reduction is the reduction of a prochiral ketone orimine.
 34. The process according to claim 32, wherein theenantioselective reduction is carried out in the presence of atetrahydro-1-methyl-3,3-diphenyl-1H,3H-pyrrolo[1,2-c][1,3,2]oxazaboroleor a 2-[(1,3,2-dioxaborolan-2-yloxy)diphenylmethyl]pyrrolidine as acatalyst.